|
 |
|
|
|
|
At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called “genes”.This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants.In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white — and never an intermediate between the two colors. These different, discrete versions of the same gene are called “alleles”.
In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent. Many organisms, including humans, have this pattern of inheritance. Organisms with two copies of the same allele are called “homozygous”, while organisms with two different alleles are “heterozygous”.
The set of alleles for a given organism is called its genotype, while the observable trait the organism has is called its “phenotype”. When organisms are heterozygous, often one allele is called “dominant” as its qualities “dominate” the phenotype of the organism, while the other allele is called “recessive” as its qualities “recede” and are not observed. Dominant alleles are often abbreviated with a capital letter, while recessive alleles are given a lowercase version of the same letter. Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once.
When a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. The outcome of these crosses can be visualized by use of a Punnett square. These observations of discrete inheritance and the segregation of alleles are collectively known as “Mendel’s first law” or the “Law of Segregation”.
A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms
